Figure 1:
Self-thermophoresis: The colloid has an engine—a hemispherical metallic coating—on board that can convert local radiation into heat via absorption. It then takes advantage of the asymmetrically created temperature profile, or more precisely, its gradient, to provide propulsion in a fixed direction relative to its own body by way of the Soret effect (see text). The active colloid will then follow a stochastic trajectory, but take its engine with it wherever it goes in the course of its motion. The mechanism will work anywhere along the trajectory so long as local radiation is available to provide “fuel” for the engine.